Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the United States. Studies have been carried out to further define the clinical spectrum of disease associated with chlamydia, to develop rapid diagnostic assays for screening and to examine the immunopathogenesis of chlamydial infection. Spectrum of chlamydial infection. We instituted clinical studies to determine the prevalence of chlamydia infection in pregnant women and to determine its association with several clinical syndromes. In a survey of 1450 pregnant women, C. trachomatis was identified by cervical culture in 14.6%. There was a significant correlation with chlamydia infection and younger age (less than 20 years of age), and with cervicitis (p less than 0.0001). In addition, chlamydia was found to be significantly associated with prematurity of the infants, and postpartum or postabortal endometritis (p less than 0.05) Development of rapid diagnostic assays. In a comparison of 2250 patient specimens, we demonstrated that detection of C. trachomatis by means of a direct immunofluorescent monoclonal antibody test is both sensitive (90%) and specific (98%) when compared to culture. In addition, it was more reliable than routine cytologic pap smear diagnosis of C. trachomatis. Detection of chlamydia by an in situ DNA hybridization method developed within our labaoratory was also found to be both sensitive (92%) and specific (80%) when compared to culture, and mzy also be used for the screening of high risk individuals. Immunopathogenesis. Using a primate model of rectal LGV infection, acute chlamydial infection was established in 10 cynomolgus monkeys and followed prospectively for 12 weeks. LBV rectal infection was xorrelated with reversal of systemic T cell lymphocyte populations and in alterations of mucosal natural killer cell populations. These changes in the immune response coincident with periods of highest infection burden, and the development of reactive lymphoid follicular hyperplasia. The above studies demonstrate the wide clinical spectrum and associated morbidity with C. trachomatis infection and provide methods for rapid screening of chlamydia and for studying its immunopathogenicity.